This invention pertains generally to fuel cells and particularly to passive air breathing polymer electrolyte fuel cells.
Polymer electrolyte fuel cells (also known as proton exchange membrane fuel cells) are well suited to low power applications of the type now typically served by conventional batteries. Such fuel cells are usually designed to use hydrogen gas as the fuel and may be designed to operate with ambient air as the oxidant. Larger fuel cells are often actively humidified, cooled, or supplied with fuel or oxygen under pressure. However, for fuel cells intended to be used as portable power sources, it is highly desirable that the operation of the fuel cell be passive, with no requirement for forcing either hydrogen fuel or air through the fuel cell.
A suitable passive fuel cell which meets the requirements for a portable power supply is shown and described in U.S. Pat. No. 5,514,486 to Wilson and U.S. Pat. No. 5,595,834 to Wilson, et al. The fuel cells shown in these patents are formed of a stack of unit cells, distributed along a common axis. The fuel cell components include a polymer electrolyte membrane, an anode and a cathode contacting opposite sides of the membrane, and fuel and oxygen flow fields contacting the anode and cathode, respectively, with the components defining an annular region therethrough along the axis that acts as a fuel flow channel. A fuel distribution manifold is mounted within the annular region to distribute fuel to the flow field in each of the unit cells. A single bolt through the annular region clamps the unit cells together. During operation of fuel cells of this type, the fuel cell consumes hydrogen supplied from a fuel source and oxygen supplied from the ambient air, and produces electricity and water. In a completely passive fuel cell of this type, there are no pumps to recirculate or remove the reactants or the reaction products. The hydrogen fuel is introduced into the hydrogen supply flow channel of the fuel cell through an inlet, but there is no outlet for the water vapor and other inert gases which accumulate within the fuel cell except through the active membranes of the cell units. Water vapor and inert gas tend to accumulate at the closed end of the hydrogen flow channel (or in the middle of the channel where hydrogen gas is fed in from both ends), with the result that the hydrogen gas is diluted in the region of the closed end (or middle) of the fuel cell and the voltages produced by the unit cells near the closed end (or middle) are consequently reduced. The inert gases most commonly include nitrogen, which can diffuse into the fuel cell through the polymer electrolyte membrane, and possibly also other impurity gases which are fed into the fuel cell with the hydrogen fuel gas.
A conventional method of removing the accumulated inert gases and water vapors from a passive fuel cell is to provide a manually operated purge valve at the closed end of the hydrogen supply channel, with the valve periodically being opened to release the inert gases and vapors. See, e.g., Larminie and Dicks, Fuel Cell Systems Explained (book), John Wiley and Sons, 2000, p. 101. The provision of a valve to be manually operated to allow purge of the fuel gases means that proper operation of the fuel cell is dependent on human intervention. If the opening of the valve to purge the system is not carried out often enough, the efficiency of the fuel cell will degrade, whereas if the system is purged too often an unnecessary volume of fuel gas will be released. The opening of the purge valve can be automated, or the hydrogen gas can be circulated using a pump, but doing so compromises the desired passive nature of the fuel cell.
In accordance with the invention, an improved passive fuel cell effectively purges water vapor and inert gases from the closed end of the fuel flow channel within the fuel cell or from the middle of the channel in a fuel cell that is fed from both ends. The purging is carried out entirely passively, without the need for moving parts, manual intervention, hydrogen circulation, or electrical control circuits. Because no active components are required, none of the electrical power output of the fuel cell need be diverted to supply power to pumps and/or controls.
The polymer electrolyte fuel cell of the present invention includes a plurality of fuel cell units arranged along a common central axis in a stack. The fuel cell units may be formed in a conventional manner, and each includes a polymer electrolyte membrane. The stacked fuel cell units have an inner periphery defining a fuel flow channel through which fuel can flow in an axial direction. An electrically conductive current collector is electrically connected to an anode of a fuel cell unit at one end of the stack and an electrically conductive current collector is electrically connected to the cathode of a fuel cell unit at another end of the stack. A fuel supply inlet is positioned at one end of the stack to direct fuel therethrough to the flow channel. At least one diffusion cell unit is mounted at the end of the stack of fuel cell units opposite to that at which the fuel inlet is mounted and at a position below and electrically insulated from the current collector at that end of the stack. The diffusion cell unit comprises at least a layer of diffuser material, and has an outer periphery exposed to the ambient atmosphere and an inner periphery that defines a continuation of the flow channel. The diffusion cell unit is formed to diffuse chiefly water vapor from the flow channel outwardly therethrough from the inner periphery to the outer periphery of the layer of diffuser material. The flow channel is closed at the end thereof that is adjacent to the diffusion cell unit. As the fuel cell operates, water vapor collects at the closed end of the flow channel at which the diffusion cell unit is located. The water vapor diffuses outwardly through the diffusion cell unit and is thereby purged from the flow channel. A plurality of diffusion cell units may be stacked together, as appropriate, to increase the rate of diffusion of the water vapor out of the fuel cell. The active fuel cell units are spaced away from the closed end of the fuel flow channel at which water vapor accumulates, and thus are exposed to significantly less water vapor than are the end cells in a conventional fuel cell stack; as a result, the overall output voltage provided from the fuel cell between the current collectors does not substantially degrade over time. For a fuel cell fed from both ends, the diffusion cell units are mounted in the middle of the stack, with an electrically conductive bridge conducting current from the active fuel cells around the diffusion cell units. Because the diffusion cell units do not contribute to the output voltage of the fuel cell, the water vapor diffusing through the diffusion cell units does not affect the operating characteristics of the fuel cell.
The diffusion cell units may be formed of a water vapor permeable membrane, such as nylon, between layers of diffuser material. A first layer of diffuser material has an inner periphery which is in communication with the flow channel and is sealed at its outer periphery from the ambient atmosphere, and a second layer of diffuser material, on the opposite side of the membrane from the first layer, has an inner periphery sealed from the fuel flow channel and an outer periphery exposed to the ambient atmosphere. Water vapor from the flow channel thus diffuses through the first diffuser layer, then through the vapor permeable membrane to the second layer of diffuser material, and thence through the second layer of diffuser material to the ambient atmosphere. The vapor permeable membrane may be formed as a polymer electrolyte membrane so that any hydrogen gas diffusing outwardly from the flow channel and oxygen in the air diffusing inwardly are combined at the membrane to produce water, thereby reducing the amount of hydrogen gas that is released by the fuel cell to the atmosphere. For such purposes, the catalyst may be formed on the side of the membrane in communication with the air, or the membrane may be catalyzed on both sides in the same manner as the normal fuel cell membrane. The membrane may also be formed of non-catalyzed materials, such as nylon film, that have the ability to pass moisture therethrough while inhibiting the flow of hydrogen and air gases therethrough.
Further objects, features, and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.